An antigen related to the major glycoprotein of xenotropic murine leukemia viruses, termed XenCSA, was shown to be a normal constituent of the cell surface of mouse lymphocytes. The level of antigen expressed on lymphocytes of different inbred strains varies considerably and appears to be regulated by several genes. In crosses between C57BL/6 and DBA/2 mice, the major regulatory gene is at or near the Fv-1 locus on chromosome 4. In crosses between DBA/2 and C57L, the influence appears to be determined by a gene on chromosome 1. Studies involving H-1 congenics suggest that another gene on chromosome 7 may influence XenCSA expression. Adult mice infected with rabies virus were shown to experience increased mortality when their humoral immune response was suppressed by treatment with antibody directed against IgM. The B cell response thus appears to be important in clearing rabies virus from the neuroparenchyma.